Geometry and structure of northern surface ruptures of the 1999 Mw=7.6 Chi-Chi Taiwan earthquake: influence from inherited fold belt structures

Surface ruptures associated with the 1999 Mw = 7.6 Chi-Chi earthquake in ce ntral western Taiwan have been characterised by mapping along the northern fault-segment. The earthquake occurred on the reactivated Chelungpu fault i n the frontal portion of the thin-skinned Taiwan fold-and-thrust belt. The N-S trending Chelungpu fault is a 90-km-long major west-verging thrust, whi ch principally slips within, and parallel to, bedding of the Pliocene Chins hui Shale. In the northern segment of the earthquake fault trace, that we n ame the. Shihkang-ShangChi fault zone, the surface ruptures turn to an E-W strike and produce a series of thrust-and-backthrust pop-ups, about 15 km l ong, forming several discontinuous subsegments distributed within a broad r egional Pliocene syncline. The northern fault segments activated during the Chi-Chi earthquake, in the area where the displacement is largest, not only display anomalous trends and a variety of mechanisms, but also raise a major problem of structural i nheritance. Detailed field investigation and kinematic analysis indicate th at the surface ruptures in the Shihkang-Shangchi fault zone are the result of dip-slip thrusting, occasionally with a minor strike-slip component. The surface ruptures emerge at the surface from bedding-parallel thrusts on bo th limbs of the regional south-plunging syncline. In the middle part of the syncline, bedding-parallel thrusts are connected by thrusts that cross-cut beds. The surface ruptures also reactivate a NE-SW trending anticline (Dia oshenshan), with west-vergent thrust on the backlimb and east-vergent backt hrust on the forelimb. This anticline is undergoing uplift by breaking thro ugh the regional N-S trending syncline. Combining GPS measurements, seismological data, and geological analyses, we propose a kinematic model with a 3-D fault surface for the 1999 earthquake in the area of the northern termination. We highlight the influence of bot h the local and regional structures (bedding parallel slip, pre-existing fa ults and folds) on the development of the earthquake rupture, and hence the role of the structural inheritance. We conclude that the south-plunging re gional Pliocene syncline in fact acts as a slip/strain guide for the northe rn termination of the Chelungpu thrust. The Pliocene Chinshui Shale, as a m ajor source of weakness within the syncline, has strongly influenced the pa ttern of slip surface during the 1999 earthquake rupture. We interpret the large vertical displacement along the northern segment as the fault rupture occurring over a surface whose radius of curvature tightens toward the nor th around the core of the syncline. (C) 2001 Elsevier Science Ltd. All righ ts reserved.